Microplastic Uptake Potential by Marine Protists During the Summer: A Case Study in Busan Coastal Waters, South Korea

Microplastic pollution poses an escalating threat to marine ecosystems, but the in situ feeding responses of protist communities to these particles remain poorly understood. This study investigated the uptake of pico-sized microplastics by protist assemblages in the coastal waters of Busan, Korea, during the summer. Three major protist groups—ciliates, heterotrophic nanoflagellates (HNF), and heterotrophic dinoflagellates—were examined through indoor incubation experiments using surface water samples spiked with 0.5 µm fluorescently labeled microspheres (FLM). In samples collected at weekly intervals, a dramatic shift was observed between late July and early August; despite a more than three-fold increase in protist growth rates (from 0.13 to 0.43 d⁻¹), the FLM uptake rates paradoxically declined by 91% (from 814 to 72 particles pn⁻¹ d⁻¹), leading to a significant drop in clearance efficiency from 81 to 11%. This disparity presumably reflected the indiscriminate filter-feeding of the dominant oligotrichs in July, whereas the decline in August was attributed to the more selective raptorial feeding of HNF, which appeared to be less effective for FLM uptake. Overall, these findings imply that microplastic removal efficiency is significantly modulated by shifts in protist community structure and their associated feeding traits.